In a conventional ion implantation apparatus, the beam line comprises an ion source, a mass separation electromagnet, a post acceleration tube (an acceleration tube provided behind the mass separation electromagnet is defined as a post acceleration tube or a back stage accelerator), and an implantation chamber as disclosed on U.S. Pat. No. 4,672,210.
An ion implantation apparatus may be a beam scan type ion implantation apparatus which scans the ion implantation ion beam by use of a scan deflection magnet or a scan electrode, or a mechanical scan type ion implantation apparatus which carries out the ion implantation by moving the implanted side (implantation chamber) relative to the ion beam. The former is chiefly used for a small current or medium current ion implantation apparatus. The latter is used for a larger current ion implantation apparatus because it is difficult to adapt the beam scan type ion implantation apparatus for a larger current. Namely, in the large current ion implantation apparatus, when the beam is scanned concentrically on one wafer, there is a problem that an amount of heat generated is increased so that the wafer is melted. For avoiding this drawback, a batch process which treats alternatively a number of wafers in a so-called batch process type mechanical scan type ion implantation apparatus is adopted.
In a medium current ion implantation apparatus in which the implantation current is lower than approximately 1 mA, it is known to provide a beam deflection electrode 7, 8, 9 behind a back stage acceleration tube 5 as disclosed in FIG. 6.15 on page 117 of "LSI Process Engineering" (written by Mr. Masatoshi Migitaka, Published by Ohm publishing Co., Ltd., on Oct. 25, 1982).
An object of providing the beam deflection electrode is to remove a neutral beam which is generated by a collision between the ion beam and residual gas existing between the post acceleration tube and a beam scan electrode.
When doubly charged ions are implanted in wafers using the ion implantation apparatus, metastable ions are generated in the beam line between the ion source and the mass separation electromagnet. When molecule ions are implanted in wafers using the ion implantation apparatus, decomposed ions are generated in the beam line between the mass separation electromagnet and the post acceleration tube.
When the doubly charged ions which are to be implanted in the wafers are, for example, P.sup.++, the metastable ions are P.sup.+ as shown in the following chemical equation and are generated between the ion source and the mass separation electromagnet decomposing dimer ions (P.sub.2.sup.+): EQU P.sub.2.sup.+ .fwdarw.P+P.sup.+
On the other hand, when the molecule ions which are to be implanted are, for example, BF.sub.2.sup.+ (boron fluoride ions), the decomposed ions are B.sup.+ or BF.sup.+ in the following chemical equations and are generated between the mass separation electromagnet and the post accelerator collisions with residual gases: EQU BF.sub.2.sup.+ .fwdarw.F.sub.2 +B.sup.+ EQU BF.sub.2.sup.+ .fwdarw.F+BF
When the metastable ions or the decomposed ions are implanted in the wafers together with the implanted ions, these ions having different energies are introduced to the wafers at the same time, and distribution of the implanted ions are varied within the wafers so that desired semiconductor elements are not obtained.
As mentioned above, the beam deflection electrode for removing the neutral beam is provided after the post acceleration tube in the small current ion implantation apparatus and the medium current ion implantation apparatus as a countermeasure for removing the metastable ions or the decomposed ions. However, in the large current ion implantation apparatus whose implantation current is larger than several mA, the beam deflection electrode cannot be used for the following reason.
Namely, large current ion beams have large repulsion forces between+ions in the beams and have a tendency to expand themselves. Stray electrons in the beams (secondary electrons generated by collision of the ions with the tube wall of the ion beam line become the stray electrons) act to suppress the expansion caused by electric charges of the ion beams themselves. If the beam deflection electrode is used, the stray electrons are attracted to the beam deflection electrode so that shapes of the ion beams can not be controlled.